When an analog video signal is received by a conventional television set having a Cathode Ray Tube (CRT), a picture is “drawn” on the CRT by sweeping an electron beam horizontally across the face of the CRT one line at a time. The sweeping of the electron beam is controlled by electromagnets in the television set. Each horizontal sweep across the face of the CRT is referred to as a scan line or simply as a line. The brightness at various points along a scan line is determined by the content of the electronic video signal. A complete picture formed from multiple scan lines is referred to as a frame.
At the end of each scan line, the electron beam is repositioned to the beginning of the next line. This is referred to as horizontal retrace. Due to the inductance of the electromagnets used to control the beam, horizontal retrace takes a certain amount of time. Accordingly, analog video signal includes not only active video regions comprising content information but also “gaps”, referred to as horizontal blanking intervals, which instruct the CRT to turn off its electron beam and perform horizontal retrace. The gaps are of sufficient duration to allow horizontal retrace to be completed. A similar mechanism is used to cause the electron beam to be repositioned from the bottom of the CRT back to the top after the last scan line has been drawn. The latter is referred to as vertical retrace.
The portion of an analog video signal which instructs the CRT to perform vertical retrace is referred to as the vertical blanking interval (VBI) region. The VBI region primarily consists of blank video lines, which follow similar conventions as active video lines but are not representative of displayed video content. VBI lines are considered to be part of an overall frame, despite the fact that they are not displayed. For example, in a television set whose CRT displays 480 scan lines of video content, each frame may actually include 45 additional, non-displayed lines which comprise the VBI region, for a total of 525 lines per frame. VBI lines may be adapted to carry various types of information associated with the video signal, such as closed captioning, content advisory, or aspect format information for example.
Images displayed on a CRT are redrawn many times per second to create the illusion of continuous motion. Continuous redrawing is performed using either interlaced or progressive scanning techniques.
In interlaced scanning, the overall frame is divided into two halves which are referred to as fields. Each field includes alternate rows (scan lines) of the frame. For example, the first field (or “top field” or “odd field”) may include odd numbered rows while the second field (or “bottom field” or “even field”) includes even numbered rows. Within the analog video signal, which may be referred to as an interlaced video signal, top and bottom fields are transmitted in sequence. A CRT receiving the signal will thus first draw the entire top field, then the entire bottom field, then the entire top field again, and so forth, in alternating fashion. Most CRT television sets employ interlaced scanning.
In progressive scanning, frames are not divided into fields. Rather, each line of a frame (whether active or VBI) is appears in sequence within the video signal, and the CRT draws each active video line of the frame in sequence before a vertical retrace occurs. The signal may be referred to as a progressive video signal in this case. Traditionally, the analog video signals received from a tower are interlaced rather than progressive video signals, however a television may generate a progressive signal from an interlaced signal in a process known as de-interlacing. Many computer monitors use progressive scanning because it may yield less “flicker” than interlaced scanning, such that a person may be able to view a picture generated with progressive scanning for longer time periods without eye fatigue than if interlaced scanning were employed.
Although most analog video signals world-wide generally follow similar conventions, analog video signals in different geographical regions may differ in such characteristics as number of scan lines per frame, visible pixels per line, horizontal scanning rate (also known as “horizontal frequency”) and vertical frame rate (also known as “vertical frequency”). For example, Table 1 illustrates the characteristics of the National Television Systems Committee (NTSC) video format prevalent in the North America and Japan as compared to the characteristics of the Phase Alternating Line (PAL) video format commonly used in Europe and the SEquentiel Couleur Avec Mémoire (SECAM) video format prevalent in France and Russia
TABLE 1NTSC and PAL format characteristicsVIDEOFORMATNTSCPAL/SECAMVisibleApproximately 480Approximately 576Lines/Frame(525 total lines)(625 total lines)VisibleDetermined by bandwidth,Determined by bandwidth,Pixels/Lineranges from 320 to 650ranges from 320 to 720Horizontal15.73415.625Rate (KHz)Vertical29.9725Frame Rate(Hz)
In most analog video formats, the VBI region may be used to carry Copy Generation Management System Analog plus Redistribution Control (CGMS-A+RC) information. CGMS-A+RC (pronounced “CGMSA plus RC”) information is an indicator of copy protection rights and redistribution control rights having two components.
The first component (CGMS-A) is a copy protection indicator which indicates whether and to what extent the associated video content may be copied. The objective of including copy protection information within an analog video signal may be to permit analog video sink devices (e.g. television sets, Video Cassette Recorders (VCRs), optical disc recorders such as Digital Versatile Disc (DVD) recorders, or Personal Computer (PC) television cards) receiving the signal to automatically take steps to prevent inappropriate copying of copyrighted content.
The second component (+RC) is a redistribution control indicator which indicates whether the video content may be redistributed, e.g., uploaded to a server on the Internet, broadcast unencrypted over a wireless network, or more generally, output over a digital output without some sort of content protection or recorded to media for long term storage. Content protection, which may constitute encryption for example, is different from copy protection, which generally precludes the making of unauthorized copies. Video may be content-protected but not copy-protected. In this case, the owner should be able to reproduce the video, e.g., by “burning” the video onto one or more recordable DVDs, but each reproduction will be viewable only by the owner (e.g. the owner may be the only one to have a key necessary to decrypt the content), rather than by anyone. Thus when redistribution control is effected, a device may automatically take steps to preclude redistribution of the video content (e.g. by automatically applying encryption before outputting a signal over a digital output of the device or automatically applying encryption before recording the content to media for long term storage).
Various VBI standards have been developed in an attempt to standardize the type and format of information carried in the VBI region of analog video signals. Some VBI standards specify carriage of both of the CGMS-A component and +RC component. For example, the CEA-805-A standard published by the Consumer Electronics Association (CEA) dictates that CGMS-A information shall be encoded using two bits and RC information shall be encoded using a single bit, as shown in Table 2 below:
TABLE 2CGMS-A and RC Bit Definitions - CEA 805-A StandardCGMS-A BitRC BitValuesCGMS-A DefinitionValueRC Definition00Copy protection not0Redistribution controlasserted (i.e.not asserted (i.e.copying permittedredistribution viawithout restriction)Internet is permitted)01Copy no more (one1No redistribution isgeneration copy haspermittedalready been made)10Copy one generationNo redistribution is(no copy has beenpermittedmade yet)11No copying ispermitted
It is noted that the CEA 805-A standard is presently undergoing revision, thus the bit definitions of Table 2 may change over time.
Other VBI standards may specify carriage of the CGMS-A component without the +RC component. Table 3 enumerates the VBI standards that are currently most prevalent. Some contention exists between the standards; efforts are being made to harmonize them. It is possible that some standards may be developed which specify carriage of only the +RC component without the CGMS-A component.
TABLE 3VBI StandardsGeographicCGMS-A (and optionally +Region ofVBI StandardRC) Repetition RateCommon UseCEA-608-BCGMS-A + RC repetitionNorth Americarate for 525i analog videocarriage = 10 to 30seconds (worst case)InternationalCGMS-A at least twiceInternationalElectrotechnicalevery 2 secondsCommission (IEC)61880 and IEC61880-2CEA-805-AType A packets: allowsNorth AmericaCGMS-A + RC everyfield/frame (deprecated).Type B packets: allowsCGMS-A + RC everyfield/frame.EuropeanAllows CGMS-A everyEurope andTelecommunicationsfield; recommends sinkAustraliaStandard Instituteresponse within 120(ETSI) EN 300 294milliseconds.IEC 62375Allows CGMS-A everyInternationalframe; recommends sinkresponse within 120milliseconds.Japanese Electronics andCGMS-A twice every twoJapanInformation TechnologysecondsIndustries Associates(JEITA) EIA-J CPR 1204Series and ARIB TR-B15
For convenience, the term “CGMS-A+RC information” will be used herein to generically refer to either or both of CGMS-A information and +RC information.
The second column of Table 3 indicates the rate at which CGMS-A+RC information is repeated within analog video signals under the relevant VBI standard indicated in column 1. CGMS-A+RC information is repeated to ensure that the information will be available to analog video sink devices regardless of the moment at which the devices lock to the signal. The repetition rate for the CGMS-A+RC information depends on the operative VBI standard. For example, as shown in Table 3, CGMS-A+RC information may be repeated as often as every field or frame or as infrequently as twice every two seconds or longer. The identity of the VBI line in which the CGMS-A+RC information is encoded may differ depending upon the operative VBI standard and the operative video format.
Despite the existence of various CGMS-A+RC formats, many providers of analog video signals (e.g. television networks) are not yet incorporating any CGMS-A+RC information within their analog video signals. Perhaps for this reason, many if not all commercially-available sink devices do not at present attempt to detect or extract CGMS-A+RC information from analog video signals.
It would be desirable for an analog video sink device to be capable of detecting CGMS-A+RC information regardless of the operative VBI standard and the operative analog video format.